1. Field of the Invention
The present invention relates to the detection of concealed electronic markers, and specifically, to a procedure and device for discriminating among a plurality of buried electronic markers.
2. Discussion of Related Art
Utility conduits are often buried underground or concealed in walls and not readily accessible. It is often necessary to locate these concealed utility conduits in order to repair and replace them. It is also important to know the location of utility conduits in order to avoid them while excavating an area. Examples of hidden utility conduits include pipelines for gas, sewage or water and cables for telephone, television or power.
There are various ways to locate concealed objects, for example, using line locators or marker locators. Line locators are appropriate when seeking electrically conductive objects, such as metallic pipelines and cables. Line locators may also be used for finding non-electrically conducting conduits when the conduit is marked with a conducting trace wire or trace tape buried along the conduit. The process of applying an AC signal to the conductor at an accessible point and detecting the resulting electromagnetic radiation is well known in the art. When an AC signal is applied, the conductor acts as an antenna radiating an electromagnetic field along its entire length.
A line locator used above ground detects electromagnetic emissions from conductors underground. A disadvantage with relying solely on the line locator device is that it may fail to identify and distinguish among various utility conduits and conductors. Additionally, line locator devices can not be used to locate non-conductive lines, such as, for example, gas lines, fiber optic lines and plastic water lines when not marked with trace wires.
Conduits may also be marked with electronic markers, either at surface level or underground. Buried electronic markers may be used to locate and identify a number of concealed objects such as cables, pipes, access points, underground stock piles, survey points and septic tanks.
Generally, electronic markers consist of two types, namely, active markers and passive markers. Active markers radiate a signal detectable at the surface; however, they require a power source. Passive markers, on the other hand, require no power source and become active when induced by an external electromagnetic field, which can be generated with a portable source.
A marker locator is a device for detecting and determining the location of concealed or buried markers. Passive markers typically include a multi-turn wire loop (coil) tuned with a capacitor to a pre-determined resonant frequency.
FIG. 1 illustrates a marker locator as operated by a location technician. Location technician 6 holds marker locator 1 directed towards ground level 7 to find the location of hidden passive markers 10 and 12. The hidden passive markers 10 and 12 can each be coded with a resonant frequency in order to identify the type of utility lines 11 and 13 that each frequency respectively marks.
Commonly, a passive marker is the combination of a wire coil and a capacitor enclosed within a non-metallic protective enclosure. The combination creates an inductance-capacitance (LC) circuit defined by an inductance developed by the wire coil and a capacitance held by the capacitor. The LC circuit operates in a resonance mode at its resonant frequency f given by the equation:                     f        =                  1                      2            ⁢            π            ⁢                          LC                                                          (                  Equation          ⁢                                          ⁢          1                )            where L is the inductance of the wire coil and C is the capacitance of the capacitor.
FIG. 2A shows an example of a ball-type passive marker. Passive marker 10 is a spherical passive marker housing three LC circuits 10A, 10B and 10C. The coils of each LC circuit 10A, 10B and 10C are positioned in orthogonal Cartesian planes such that the three tuned circuits produce a uniform radio frequency (RF) field.
FIG. 2B shows a disk-type passive marker. Passive marker 12 is a flat passive marker housing a single LC circuit 12A with the coil positioned in the horizontal X-Y plane.
FIG. 3 shows the electrical schematic diagram of a single LC circuit. The coil acts as an inductor 16, and is connected in parallel with a capacitor 18 to form a resonant tank circuit 14. The frequency f of the passive marker is set by the resonant frequency of the passive LC circuit, which can be tuned to a preset value.
Different types of utility lines are each associated with unique resonate frequency values. Markers with different resonant frequencies may also be colored for quick identification when installed. Six distinct frequency/color combinations are commonly used: 77.0 kHz (Orange/Black) for Canadian telephone and Cable TV; 83.0 kHz (Yellow) for Gas; 101.4 kHz (Orange) for Telephone; 121.6 kHz (Green) for Sanitary/Waste water; 145.7 kHz (Blue) for Water; and 169.8 kHz (Red) for Power. Of course, these frequencies (and colors) have been designated by conventional use and are not meant to be restrictive.
Though passive electronic markers have several advantages over tracing wires, they are still subject to certain limitations. One such problem is the time consumed by separate searches for each type of marker. Another such problem is the “neighbor detection” problem where emissions of marker-types not being searched for overwhelm the receiver producing false-positive indications. A similar problem is the “near-far” problem where emissions from nearby markers can override signals from the farther placed marker possibly producing an erroneous marker indication.
In light of the foregoing description, it would be desirable to devise an improved method for locating markers. It would also be desirable to reduce the occurrence of erroneous marker indications. It would be further advantageous if a method existed that could facilitate detection of all markers in a given area more quickly than is conventionally known.